I hope this is the right forum... I was just wondering, what is the probability that open rotor engines will replace jet turbofans in the future? I have been doing a lot of reading on the subject, and from my understanding they are more fuel efficient but also slower and noisier than conventional turbofan engines.. they're also quite hideous looking in design. I personally don't like them, and would hate to see them replacing the turbofan jet engine that we all know and love. Would anyone care to comment on this?

Well, I think the chances WERE good, up until recently.
The open rotor, as you're already aware, offers some distinct advantages, and Rolls Royce was rumors to be kicking the idea round again. But there are also distinct challenges to this design; otherwise, in the thirty years since this idea debuted, we'd have seen the technology employed.

BUT- now, with Pratt & Whitney's geared turbofan design, I think that the wave of the near future will be in designs like this. That is, a decoupled fan, maybe even two stages if engineers find it attractive, geared to the low pressure turbine or perhaps their own turbine stage.

The geared turbofan has a much higher bypass ratio, resulting in more fuel efficiency. Surely then it's just a case of increasing the bypass ratio in future designs without resorting to the use of those loud and ugly open rotor engines. I know the new geared turbofan engines on the Bombardier C-series and A320neo will be about 12% more efficient, so hopefully they'll keep updating these engine types until the open rotor has no advantages over the turbofan anymore. Besides the obvious design challenges of the open rotor, I don't think it would bode too well with the flying public, who are comfortable with the current turbofan design, and have been for decades.

Not necessarily a higher bypass ratio. The main feature is the gear allows the turbine spin much faster than the fan (3:1 on the current PW designs IIRC) so that both spin at more efficient rates and thus require less turbine stages to turn the fan (and in turn less compressor stages too). Because the fan spins slower it's easier to make it bigger too (blade tip clearance is easier to maintain, etc).

Quoting FlyingBattery26 (Reply 2):Surely then it's just a case of increasing the bypass ratio in future designs without resorting to the use of those loud and ugly open rotor engines

There is a limiting point where the weight of the bigger blades and containment system and longer/stronger landing gear doesn't earn its keep.

However we're in the early days of GTF and there were various conservative steps taken to get it to market.

Quoting FlyingBattery26 (Thread starter):I was just wondering, what is the probability that open rotor engines will replace jet turbofans in the future?

Quite frankly, who knows? Even under the best situation we are still at least 10 years out before OR's are "practical" enough to be ready for consideration for commercial use. And, even then, they would need a purpose designed airframe... and if that airframe were to be the two major OEM's next all-new NB then we are looking at an EIS not earlier than 2030.

Much more realistic are the earlier described GTF dominating the mid-term engine market. Both PW and GE will pour money into design improvements, some of which will only be available in 5-10 years.

Further enhancing GTFs is the probability of some form of hybrid propulsion. Boeing last year came out with their Sugar Volt" design for NASA and Aviation Week unveiled some older Boeing patents utilizing hybrid propulsion (they also patented a design for OR powered aircraft with an upper mid-fuse engine placement.)

So, to sum up... still way to early to specify "future" propulsion system (though we may speculate).

Quoting Revelation (Reply 3):The main feature is the gear allows the turbine spin much faster than the fan (3:1 on the current PW designs IIRC) so that both spin at more efficient rates and thus require less turbine stages to turn the fan (and in turn less compressor stages too). Because the fan spins slower it's easier to make it bigger too (blade tip clearance is easier to maintain, etc).

Except perhaps that with a GTF, the fan is shrouded or cowled. (always?)
In gear configuration, the fan can be displaced (like the turboprop), so that could afford more ground clearance, or more (or less?) aerodynamic prop wash.

So open rotors have a future, just not on widebodies. They are most attractive in the 300nm to 1000nm missions. On missions longer than that, the added flight time adds costs and detracts from the experience. On shorter missions, the turboprop has cheaper (lighter, simpler) economics. So there is a market. But not for all missions. No one will be flying LAX-JFK on an Open Rotor. But STN-HHN is a very ideal route.

Quoting AA737-823 (Reply 1):BUT- now, with Pratt & Whitney's geared turbofan design, I think that the wave of the near future will be in designs like this. T

Pratt is now developing 4.5:1 ratio gearboxes (PurePower so far have all been 3:1 ratio gearboxes) which will compete with Open rotors in many markets.

Ugly has nothing to do with it. Loud is the concern, which is why there will be a cruise Mach speed limit. But without the shroud, the Open rotor is much lighter than the GTF. The shroud does help cruise efficiency, but the Open rotor has a tremendous climb fuel burn advantage. So it will all come down to the market and 300nm to 1000nm is a HUGE fraction of narrow body flights. It will depend on each airline's flight distribution. e.g., B6 wouldn't consider an open rotor by U2 and FR should.

I'm a huge fan of the GTF. It has its niches. But so does the open rotor.

Quoting rampart (Reply 5):Except perhaps that with a GTF, the fan is shrouded or cowled. (always?)

Quoting lightsaber (Reply 6):Ugly has nothing to do with it. Loud is the concern, which is why there will be a cruise Mach speed limit. But without the shroud, the Open rotor is much lighter than the GTF.

There's also a safety issue. A blade-off event in a turboprop can crash the entire aircraft. An open rotor spins even faster and a blade-off would be absolutely devastating to anything in its path. The engine itself can be designed to separate from the aircraft in the event of an unbalance event, but that's also a huge can of worms (sucks to be the poor schmuck under the engine when it comes down). Almost certainly, certification will require that the rotors be aft of any critical aircraft components.

The noise is really the big issue. At a speed of M=0.75, one only need add M=0.25 to the tips (and they're rotating, so that's easy to do) and shockwaves will form. Supersonic propellers have been tried and were immediately abandoned because they would blow peoples' ears out and cause seizures. (Google the XF-84H).

Quoting DocLightning (Reply 8):
There's also a safety issue. A blade-off event in a turboprop can crash the entire aircraft. An open rotor spins even faster and a blade-off would be absolutely devastating to anything in its path. The engine itself can be designed to separate from the aircraft in the event of an unbalance event, but that's also a huge can of worms (sucks to be the poor schmuck under the engine when it comes down). Almost certainly, certification will require that the rotors be aft of any critical aircraft components.

How would a blade separation for an open rotor be worse than an uncontained engine failure in a normal turbine? Far as I know all large civil airliners are designed assuming that the blades have essentially infinite energy, and systems placed and designed accordingly. At least they are now.

Quoting discovery1 (Reply 10):How would a blade separation for an open rotor be worse than an uncontained engine failure in a normal turbine?

An uncontained failure in a normal engine is going to go one of two ways: out the front or out the back. The nacelle will hold in a blade that gets tossed. Open rotor of course has no nacelle and therefore nothing to stop a blade that decides to leave.

Quoting BMI727 (Reply 12):
An uncontained failure in a normal engine is going to go one of two ways: out the front or out the back. The nacelle will hold in a blade that gets tossed. Open rotor of course has no nacelle and therefore nothing to stop a blade that decides to leave.

Basically must. not. happen. Uncontained fan failures are not permitted and they are part of the design certification process.

Rotors and turbine blades must be designed and maintained so that they will. not. fail. The few cases in which they have done so have often been devastating (UA 232). That QF A380 had a guardian angel watching it. If one of those disk fragments had gone through the fuselage...

Quoting lightsaber (Reply 14):However, Airbus has a chance. Will they do it? Obviously not before 2018 (Airbus engineering is too stretched until then.)

Aside from an OR not being ready by 2018, IMO, I do not see A nor B going for OR... whenever OR is ready for "primetime". As you pointed out, They are most attractive in the 300nm to 1000nm missions. So A or B would have to develop a separate program alongside their next all-new NB design... not something that is really feasible.

Quoting discovery1 (Reply 10):How would a blade separation for an open rotor be worse than an uncontained engine failure in a normal turbine? Far as I know all large civil airliners are designed assuming that the blades have essentially infinite energy

I think you're confusing blade events and rotor failures. An individual blade-out is containable and as far as I know, must be contained to certify. There's a *lot* more energy in the rotor disks; a rotor burst is not reasonably containable, so a disk failure must be improbable to the point of almost-never-happening.

Quoting DocLightning (Reply 8):Almost certainly, certification will require that the rotors be aft of any critical aircraft components.

Is this really true about being aft? I would think that blade-out from an on-wing installation of open rotor, piercing the fuselage may be (relatively speaking) not as bad a blade-out from a rear installation taking out a part of the tail. I.e., it's not component damage but structure damage that is more devastating in the aft installation. (Aircraft is basically crashing in the later scenario)

Quoting planemaker (Reply 4): Even under the best situation we are still at least 10 years out

The technology has been "10-years away" basically from inception in late 1980s. I am still skeptical. Safety is crucial, and so is the noise in many parts of the world. The fact that RR moved to cooperate with PW on future geared fan projects speaks volumes of the fate of that technology.

Quoting ChicagoFlyer (Reply 18):Is this really true about being aft? I would think that blade-out from an on-wing installation of open rotor, piercing the fuselage may be (relatively speaking) not as bad a blade-out from a rear installation taking out a part of the tail. I.e., it's not component damage but structure damage that is more devastating in the aft installation. (Aircraft is basically crashing in the later scenario)

If it's mounted such that it extends aft of the tail, then there is no problem. A blade off will travel forward of the engine very briefly but very quickly find itself aft of the aircraft.

Quoting Revelation (Reply 3):Not necessarily a higher bypass ratio. The main feature is the gear allows the turbine spin much faster than the fan (3:1 on the current PW designs IIRC) so that both spin at more efficient rates and thus require less turbine stages to turn the fan (and in turn less compressor stages too). Because the fan spins slower it's easier to make it bigger too (blade tip clearance is easier to maintain, etc).

In the AIN article PWVP states that they will be able to increase the ratio to 5:1 from the 3:1 that you recalled correctly.

Quoting Revelation (Reply 3):However we're in the early days of GTF and there were various conservative steps taken to get it to market.

In addition, PW will catch up to GE on the materials side. So, as PWVP said, why go into the OR arena when they match the benefits with GTF without any of the considerable drawbacks.

Short to mid term GTFs will dominate... and then some form of electric hybrid propulsion (there are a couple of approaches being researched by OEMs) probably will be incorporated.

Quoting planemaker (Reply 23):In addition, PW will catch up to GE on the materials side. So, as PWVP said, why go into the OR arena when they match the benefits with GTF without any of the considerable drawbacks.

I happen to believe that. But if Pratt ignores the OpenRotor, it will estabilish itself. Aircraft and engines are a marathon, not a sprint. Perhaps more like a death march... One has to look at the serious potential of disruptive technology and address the advantages of your technology, or one will lose the advantage.

Quoting planemaker (Reply 23):Short to mid term GTFs will dominate... and then some form of electric hybrid propulsion (there are a couple of approaches being researched by OEMs) probably will be incorporated.

Eventually, I believe we'll have fuel cells in the cargo hold powering electric propellers. But that is quite a few decades in the future.

Well, you would know. I believe the GTF was a 22 year effort... and one could make the case that if PW didn't jump on board the CSeries the GTF still wouldn't be hitting the market.

Quoting lightsaber (Reply 24):One has to look at the serious potential of disruptive technology and address the advantages of your technology, or one will lose the advantage.

Absolutely! And that is the thing... over the next 5, 10, 15 years there will be advancements in materials science, modelling, manufacturing, etc. But, right now, I believe that PW is holding the best hand going forward.

Quoting lightsaber (Reply 24):Eventually, I believe we'll have fuel cells in the cargo hold powering electric propellers. But that is quite a few decades in the future.

Electric propulsion, and there are several routes it could go including fuel cells that you highlighted, could be much sooner than we would anticipate at present. I believe that in less than 10 years there will be a societal shift that will take climate change seriously and that will force major, and disruptive, changes to the carbon economy.

Quoting BMI727 (Reply 12):An uncontained failure in a normal engine is going to go one of two ways: out the front or out the back. The nacelle will hold in a blade that gets tossed.

There have been a number of instances where a fan blade was ejected through the nacelle and into the fuselage, it does happen. I believe the total containment has been something that is as new as the development of the engines for the 777 in the mid 90s with use of what I expect was carbon fiber in the nacelle areas that would be next to the fan blades. Back in the early-mid 90s a Rich International L-1011 here in Fairbanks was taxiing for takeoff at FAI when on of its RB211s had what can only be called an uncontained engine failure and threw fan blades through the nacelle and into the fuselage in many more than one spot both into the cargo area and passenger cabin but luckily nobody was injured.
Also the Delta MD88 engine failure in Jacksonville killed a passenger seated in the rear of the cabin next to the engine.

See some of my comments on the 90 seat turboprop thread. I find the UDF design novel and still get a kick at seing the works spin (I'm ommitting the P word here). IN the past 30 years technology has vastly improved so I can imagine a UDF design matching the performance of the jets we know and love. This is going out there but I could imagine an HSCT with the massive wing mounted jet engines and these things mounted aftward for subsonic flight including climb and descent.

Quoting planemaker (Reply 25):Electric propulsion, and there are several routes it could go including fuel cells that you highlighted, could be much sooner than we would anticipate at present. I believe that in less than 10 years there will be a societal shift that will take climate change seriously and that will force major, and disruptive, changes to the carbon economy.

The big issue that electric propulsion is going to need to get around is weight. There are electric motors today powerful enough to drive a large turbofan. Here's one of them:

They're so big that right now they are used in ships. Now, new materials, advances in new low-resistance conductors, etc. will bring the weight now, but as long as electric motors need huge windings and magnets...

Quoting DocLightning (Reply 28):They're so big that right now they are used in ships. Now, new materials, advances in new low-resistance conductors, etc. will bring the weight now, but as long as electric motors need huge windings and magnets...

I believe that we'll see intermediate steps before we see a fully "pure" electric turbofan for the reason you state. First up could be a hybrid along the lines of this GE hFan concept that can operate in just gas turbine, all-electric or combined modes. It would takes off and climbs in combined mode and then cruise in either mode depending on mission profile.

Of course, GE has a large electric motor division and is doing a lot of R&D. Last July they unveiled a prototype electric motor for hybrid cars that was half the size for the same energy output but 3-5% more efficient. So we can expect continuing advancement in all areas over the next 10 20 years.

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